Display apparatus having a two-terminal device including a zinc sulfide layer and method for producing the same

ABSTRACT

A display apparatus including a plurality of pixel electrodes arranged in a matrix on a first substrate; a scanning line for sending a signal to the plurality of pixel electrodes for driving the plurality of pixel electrodes; a switching device for receiving the signal from the scanning line and switching each of the plurality of pixel electrodes into one of a conductive state and a non-conductive state in accordance with the signal; a counter electrode on a second substrate opposed to the first substrate; and a display medium layer sandwiched between the first substrate and the second substrate. The switching device includes a two-terminal element having a first electrode which is a part of the scanning line; the zinc sulfide layer on the first electrode, said zinc sulfide layer having an I-V characteristic expressed by a continuous curve; and a second electrode located on the zinc sulfide layer and electrically connected to the pixel electrode.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a display apparatus using atwo-terminal device as a switching device and a method for producing thesame.

2. Description of the Related Art

A representative display apparatus replacing CRTs which have been usedfor a long time is a liquid crystal display apparatus (hereinafter,referred to as the "LCD apparatus"). LCD apparatuses use a liquidcrystal layer including liquid crystal molecules as a display medium.Letters and images are displayed by applying a voltage to the liquidcrystal layer in order to cause changes in the electrooptic propertiesof the liquid crystal molecules. In order to display high quality imagesby providing pixels at a high density, each pixel is supplied with anonlinear active element (switching device) for driving the LCDapparatus. This system of driving is referred to as the "active matrixdriving system". The switching devices are mainly available in twotypes: two-terminal devices such as MIM (metal insulator metal)elements, diodes, and varistors; and three-terminal devices such as TFTs(thin film transistors) and MOS-FETs (metal oxide semiconductor fieldeffect transistors).

The three-terminal devices function as switching devices and aresuitable for displaying an image having various tones, for whichdifferent pixels are used for each of the different tones. However, thethree-terminal devices have some inconveniences in that the complicatedproduction process including repetition of exposure to light can easilycause defects in the obtained devices, thus resulting in low productionyields. The two-terminal devices, which have a simpler structure thanthat of the three-terminal devices, are produced by a simpler methoddue, for example, to fewer steps of masking required. Accordingly, theproduction yield of the two-terminal devices is higher than that of thethree-terminal devices. For this reason, methods for driving pixelsusing the two-terminal devices, especially by utilizing a nonlinear partof the operating characteristics of the two-terminal devices, have beenactively researched and developed.

There are mainly two methods to use the two-terminal devices in adisplay apparatus. One is to utilize the nonlinearity of the capacitanceof the two-terminal devices, and the other is to utilize thenonlinearity of the electric resistance of the two-terminal devices.

Methods to utilize the nonlinearity of the capacitance of thetwo-terminal devices for an LCD apparatus are described by Grabmair etal. in Mol. Cryst. Liq. Cryst. 15 (1971) and by Tannas in SID'73 Symp.Digest (1973). In these publications, ferroelectric liquid crystals areused. However, such methods have not been put into practical use. Thereasons include (1) a large driving voltage is required; and (2) sincethe dielectric constant of the ferroelectric liquid crystals largelydepends on temperature, the characteristics of the resultant LCDapparatus also largely depend on temperature.

Utilization of the nonlinearity of the electric resistance is describedby Lechner in Proc. IEEE 59. (1971), by Castleberry in IEEE. Trans.Electron Devices ED-26 (1979), and by Baraff in IEEE. Trans. ElectronDevices ED-28 (1981). Lechner uses a diode, Castleberry uses a varistorformed of ZnO, and Baraff uses an MIM element.

A display apparatus using an MIM element includes the followingadvantages.

(1) The area occupied by an MIM element in one pixel is smaller than thearea occupied by a TFT. Accordingly, the ratio of an area of pixels withrespect to an image plane (the opening ratio) is larger than in the caseof a TFT.

(2) Since the scanning lines and the signal lines do not intersect eachother on a substrate as they do in the case of a TFT, there are fewerline defects caused by insufficient insulation at the intersections.

(3) The production process is simpler, e.g., the number of masking stepsis smaller than in the case of a TFT. Accordingly, the production yieldis higher.

(4) There is no electric current excited by light, which is a problem inTFTs using amorphous silicon, diodes and the like. This eliminates thenecessity of shielding the MIM element from external light.

Briefly referring to FIGS. 26 and 27. A conventional display apparatus50 will be described. FIGS. 26 and 27 show an example of a conventionaldisplay apparatus 50 including a conventional MIM element 58 as aswitching device. FIG. 26 is a top view of a pixel and the vicinitythereof in the display apparatus 50; and FIG. 27 is a cross sectionalview of the display apparatus 50 shown in FIG. 26 looking along sectionline 27'--27' in FIG. 26.

The display apparatus 50 includes an insulating substrate 51 formed ofglass or the like. A scanning line 52 formed of tantalum (Ta) is on atop surface of the substrate 51. An electrode 56 is branched from thescanning line 52 perpendicularly to the scanning line 52. A surface ofthe scanning line 52 and a surface of the electrode 56 are anodized soas to be an insulation layer 53. Specifically, the insulation layer 53is formed of Ta₂ O₅. A rectangular metal layer 54 formed of Ta or thelike is on the substrate 51, covering the insulation layer 53. The metallayer 54 is arranged in a direction so as to cross the electrode 56. Agenerally rectangular pixel electrode 55 as is shown in FIG. 26 is onthe substrate 51, covering two ends of the metal layer 54.

The MIM element 58 includes a three-layer structure including theelectrode 56, the insulation layer 53, and the metal layer 54. Theelectrode 56 acts as a first metal layer, the insulation layer 53 actsas an active layer, and the metal layer 54 acts as a second metal layer.Display apparatuses including such a two-terminal device havingnonlinear characteristics are now produced as commercial products.

Japanese Patent Publication Nos. 61-32673 and 61-32674 each disclose anLCD apparatus including an MIM element as the two-terminal device.

In order to use the above-described two-terminal device as a switchingdevice, the voltage V applied to an active layer (the insulation layerin the case of an MIM element) between the first metal layer and thesecond metal layer and the current I flowing between the two metallayers in accordance with the voltage V should have the following I-Vcharacteristic:

(1) The current I rises steeply in accordance with the rise of thevoltage V. In other words, the I-V characteristic has a satisfactorysteepness.

(2) The absolute value of the current I depends on the absolute value ofthe applied voltage V and is independent of the polarity (+ or -) of thevoltage V. In other words, the I-V characteristic has a satisfactorysymmetry in the polarity of the voltage.

For obtaining an image which is entirely uniform in quality, all thepixels in the image plane should have an I-V characteristic excellentboth in steepness and symmetry.

FIG. 28 is a graph illustrating a curve representing the I-Vcharacteristic of a conventional MIM element used, for example, in thedisplay apparatus 50. As is apparent from FIG. 28, the I-Vcharacteristic does not show the above-mentioned symmetry. This fact isattributable to the following reasons (1) and (2) below.

(1) The first metal layer is in contact with the insulation layer formedby anodizing the first metal layer. The second metal layer is formed onthe insulation layer by sputtering or CVD (chemical vapor deposition).The interface between the first metal layer and the insulation layer andthe interface between the second metal layer and the insulation layerare formed by different processes from each other. Accordingly, thesetwo interfaces are in different states.

(2) While the first metal layer is anodized, impurities go into theinsulation layer, thus causing the interface between the first metallayer and the insulation layer and the vicinity thereof to have adifferent dopant concentration from that at the interface between thesecond metal layer and the insulation layer and the vicinity thereof.

As is described above, the state of the interface between the firstmetal layer and the insulation layer and the state of the interfacebetween the second metal layer and the insulation layer influence theI-V characteristic of the MIM element. Accordingly, types of metalusable for the MIM element are restricted. Conventionally, where Ta isused for the first metal layer and Ta₂ O₅ is used for the insulationlayer, the second metal layer is formed of Cr, Ta or Ti. If the secondmetal layer is formed of Al, or ITO (indium tin oxide), the symmetry ofthe I-V characteristic is significantly deteriorated.

The I-V characteristic of the MIM element is expressed by Equations (1),(2) and (3) by Poole-Frenkel current. ##EQU1## where q is the electriccharge, n is the carrier density, μ is the mobility, φ is the depth ofthe trap, d is the thickness of the insulation layer, T is thetemperature, k is the Boltzmann constant, and .di-elect cons. is thedielectric constant.

As is apparent from Equation (1), β which is expressed by Equation (3)indicates the steepness of the I-V characteristic. It is preferable toobtain the highest possible value for β. For example, the value of β isapproximately 3 to 4 inclusive in the MIM element including theinsulation layer formed of Ta₂ O₅, while the value of β in a varistor,which is a type of the two-terminal device, is approximately 7 to 8inclusive. This signifies that the MIM element using Ta₂ O₅ for theinsulation layer is inferior to the varistor in the I-V characteristicin the steepness.

It is apparent from Equation (3) that the value of β depends on thethickness d of the insulation layer. Therefore, the I-V characteristicchanges in accordance with any slight difference in the thickness d.This causes dispersion among the curves representing the I-Vcharacteristic of a plurality of MIM elements for driving respectivepixels. As a result, the pixels are put into different display states,thus deteriorating the display quality of the whole image.

Conventional two-terminal devices, for example, the MIM element, alsohave the following problems in the production process.

In the case where the active layer (for example, the insulation layer 53in the case of the MIM element 58 in FIG. 27) insufficiently covers thefirst metal layer (the first electrode) 56, especially on a tapered sidesurface 57 of the first metal layer 56 and the vicinity thereof,insulation breakdown occurs in the MIM element 58. In thisspecification, the vicinity of the tapered side surface 57 includes anedge formed at the junction of a top surface 59 of the first metal layer56 and the tapered side surface 57 and also includes an edge formed atthe junction of the tapered side surface 57 and the interface betweenthe first metal layer 56 and the base substrate 51.

For example, in the case where the first metal layer 56 is patterned byetching, insufficient insulation of the tapered side surface 57 and thevicinity thereof adversely influences the performance of the MIM element58, thereby causing defects and deterioration in the MIM element 58.

In order to solve these problems, some proposals have been made.

One of the proposals is made in Japanese Laid-Open Patent PublicationNo. 1-270027, which discloses an MIM element as is shown in FIG. 29.

As is shown in FIG. 29, a display apparatus 60 includes a substrate 61.A first metal layer 62 is on a top surface of the substrate 61. Asurface of the first metal layer 62 is anodized so as to be aninsulation layer 63 (active layer). An insulating intermediate layer 66is on the substrate 61, covering the insulation layer 63. Theintermediate layer 66 has a hole 67 reaching a top flat surface 68 ofthe insulation layer 63. A second metal layer 64 is on the intermediatelayer 66. A pixel electrode 65 is on the intermediate layer 66, coveringan end of the second metal layer 64. The second metal layer 64 is incontact with the insulation layer 63 through the hole 67 and also is incontact with the pixel electrode 65. By such a structure, an MIM element70 includes a flat portion of the first metal layer 62, but excludes atapered side portion 69 and the vicinity thereof in the first metallayer 62. Accordingly, the performance of the MIM element 70 isprotected against adverse influences caused by insufficient insulationof the tapered side portion 69 and the vicinity thereof.

Japanese Laid-Open Patent Publication No. 5-72570 discloses a method forproducing an MIM element 80 as is shown in FIG. 30. In FIG. 30,identical elements with those in FIG. 29 bear identical referencenumerals therewith.

According to this method, after the first metal layer 62 is formed in apattern on a top surface of the substrate 61, the insulation layer 63(active layer) and the intermediate layer 66 are formed by radiatinglight using the first metal layer 62 as a mask (a photoresist method)from the side of a bottom surface of the substrate 61. As a result, theinsulation layer 63 is formed only on a flat top surface 71 of the firstmetal layer 62. Due to such a structure, the problem of insulationbreakdown due to insufficient insulation of a tapered side portion andthe vicinity thereof is solved.

However, in order to realize the structure shown in FIG. 29, in whichthe second metal layer 64 is in contact with the insulation layer 63 andalso is in contact with the pixel electrode 65, a part of theintermediate layer 66 and a part of the insulation layer 63 shouldselectively be etched away. If the insulation layer 63 is formed of amaterial having a low resistance against chemicals used as an etchant,it is difficult to realize the structure shown in FIG. 29.

In the case where the photoresist method is used as in FIG. 30, themanufacturing precision will be quite limited.

Due to these reasons, it is difficult to put the display apparatus 60shown in FIG. 29 and the MIM element 80 into practical use.

In the structure shown in FIG. 29, the insulation layer 63 is formed inareas other than the MIM element 70. Accordingly, the leak current hasundesirable increases, so the impedance undesirably changes inaccordance with the change in the applied voltage. For these reasons,the display quality declines.

SUMMARY OF THE INVENTION

A display apparatus according to the present invention includes aplurality of pixel electrodes arranged in a matrix on a first substrate;a scanning line for sending a signal to the plurality of pixelelectrodes for driving the plurality of pixel electrodes; a switchingdevice for receiving the signal from the scanning line and switchingeach of the plurality of pixel electrodes into one of a conductive stateand a non-conductive state in accordance with the signal; a counterelectrode on a second substrate opposed to the first substrate; and adisplay medium layer sandwiched between the first substrate and thesecond substrate. The switching device includes a two-terminal elementhaving: a first electrode which is a part of the scanning line; the zincsulfide layer on the first electrode, the zinc sulfide layer having anI-V characteristic expressed by a continuous curve; and a secondelectrode located on the zinc sulfide layer and electrically connectedto the pixel electrode.

In one embodiment of the invention, the zinc sulfide layer has athickness of 10 nm to 1 μm.

In one embodiment of the invention, the zinc sulfide layer is on anentire surface of the first substrate, covering the scanning line.

In one embodiment of the invention, the zinc sulfide layer is on aspecified area of the first electrode excluding a side surface and thevicinity thereof which includes an edge formed at the junction of a topsurface and the side surface of the first electrode and an edge formedat the junction of the side surface and the interface between the firstelectrode and the first substrate.

In one embodiment of the invention, the first electrode is branched fromthe scanning line.

In one embodiment of the invention, the first electrode is a specifiedarea within the scanning line.

In one embodiment of the invention, the second electrode and the pixelelectrode are formed of identical materials.

In one embodiment of the invention, the second electrode is branchedfrom the pixel electrode.

In one embodiment of the invention, the second electrode is a specifiedarea within the pixel electrode.

In one embodiment of the invention, the display apparatus furtherincludes an insulation layer sandwiched between the pixel electrode andthe first substrate except for the specified area on which the zincsulfide layer is located.

In one embodiment of the invention, the display apparatus furtherincludes a first insulation layer sandwiched between the first electrodeand the zinc sulfide layer.

In one embodiment of the invention, the first insulation layer isobtained by anodizing the first electrode.

In one embodiment of the invention, the display apparatus furtherincludes a second insulation layer sandwiched between the zinc sulfidelayer and the second electrode.

In one embodiment of the invention, the second insulation layer has ahole, through which the second electrode and the zinc sulfide layer areelectrically connected with each other.

In one embodiment of the invention, the display apparatus furtherincludes a first insulation layer sandwiched between the first electrodeand the zinc sulfide layer and a second electrode sandwiched between thezinc sulfide layer and the second electrode.

In one embodiment of the invention, the first insulation layer and thesecond insulation layer are formed of identical materials.

In one embodiment of the invention, the hole has an opening having anarea from 10 μm² to 1,000 μm² inclusive.

In one embodiment of the invention, the zinc sulfide layer includesdopant.

In one embodiment of the invention, the dopant is selected from thegroup consisting of manganese, copper, rare earth elements, compoundsincluding a rare earth element, and the III-group elements.

In one embodiment of the invention, the zinc sulfide layer is formed soas to have a composition ratio expressed by 1>x>0.5 where the zincsulfide layer has a composition expressed by Zn_(x) S.sub.(1-x).

In one embodiment of the invention, the first insulation layer is formedof a substance selected from the group consisting of nitrogen compoundsand silicon oxide.

In one embodiment of the invention, the second insulation layer isformed of a substance selected from the group consisting of nitrogencompounds and silicon oxide.

In one embodiment of the invention, the first insulation layer and thesecond insulation layer are each formed of a substance selected from thegroup consisting of nitrogen compounds and silicon oxide.

In one embodiment of the invention, at least one of the pixelelectrodes, the first electrode and the second electrode is formed of atransparent conductive layer.

In one embodiment of the invention, the first insulation layer includesa plurality of insulation layers formed of different substances.

In one embodiment of the invention, the second insulation layer includesa plurality of insulation layers formed of different substances.

In one embodiment of the invention, both of the first insulation layerand the second insulation layer includes a plurality of insulationlayers formed of different substances.

In one embodiment of the invention, the pixel electrode is formed of asubstance selected from the group consisting of Al, Ag, Cr, Ni, Cu, Tiand alloys thereof.

In one embodiment of the invention, the second insulation layer has acorrugated surface.

In one embodiment of the invention, the second insulation layer isformed of a color photoresist.

In one embodiment of the invention, the display medium is a liquidcrystal.

In one embodiment of the invention, the display medium is a White-Taylorguest-host liquid crystal.

In one embodiment of the invention, the display apparatus furtherincludes a plurality of insulation layers on the first substrate forsupplying the first substrate with an insulating property.

In another aspect of the invention, a method for producing a displayapparatus including a two-terminal element includes: step (a) of forminga conductive layer on an insulating substrate; step (b) of patterningthe conductive layer into a specified pattern so as to form a scanninglayer having a first electrode; step (c) of forming a zinc sulfide layerentirely on the substrate to cover the scanning line, the zinc sulfidelayer having such a specified thickness as to have an I-V characteristicexpressed by a continuous curve; step (d) of forming a second electrodeon the zinc sulfide layer, the second electrode being superimposed overat least a portion of the first electrode; step (e) of forming a pixelelectrode on the zinc sulfide layer in such a pattern as to beelectrically connected to the second electrode. The two-terminal elementincludes the first electrode, the zinc sulfide layer, and the secondelectrode.

In one embodiment of the invention, steps (d) and (e) are performedsimultaneously, and the second electrode and the pixel electrode areformed by patterning another second conductive layer.

In one embodiment of the invention, the method further includes at leastone of step (f) of forming a first insulation layer on the substratebetween step (b) and step (c); and step (g) of forming a secondinsulation layer on the substrate between step (c) and step (d).

In one embodiment of the invention, step (g) includes step (h) offorming a hole for electrically connecting the second electrode and thezinc sulfide layer in the second insulation layer, the hole being formedby a photolithographic method, by which light is radiated toward thesubstrate from the side of a surface thereof having no scanning line andusing the scanning line as a mask.

In one embodiment of the invention, the zinc sulfide layer is heatedbetween step (c) and step (d) at a temperature which is higher than thetemperature of the substrate for forming the zinc sulfide layer in step(c).

In one embodiment of the invention, the zinc sulfide layer is heatedafter step (d) at a temperature which is higher than the temperature ofthe substrate for forming the zinc sulfide layer in step (c).

In one embodiment of the invention, the zinc sulfide layer is heatedbetween step (c) and step (g) at a temperature which is higher than thetemperature of the substrate for forming the zinc sulfide layer in step(c).

In one embodiment of the invention, the zinc sulfide layer is heated inan atmosphere selected from the grope consisting of a vacuum atmosphereand a sulfur atmosphere.

In one embodiment of the invention, an impurity is implanted into thezinc sulfide layer in step (c).

In one embodiment of the invention, the impurity is selected from thegroup consisting of manganese, copper, rare earth elements, compoundsincluding a rare earth element, and the III-group elements.

In one embodiment of the invention, the zinc sulfide layer is formed soas to have a composition ratio expressed by 1>x>0.5 where the zincsulfide layer has a composition expressed by Zn_(x) S.sub.(1-x).

In one embodiment of the invention, at least one of the pixelelectrodes, the first electrode and the second electrode is formed of atransparent conductive layer.

In one embodiment of the invention, the pixel electrode is formed of asubstance selected from the group consisting of Al, Ag, Cr, Ni, Cu, Tiand alloys thereof.

In one embodiment of the invention, the zinc sulfide layer has athickness of 10 nm to 1 μm.

In still another aspect of the invention, a method for producing adisplay apparatus including a two-terminal element includes step (a) offorming a conductive layer on an insulating substrate; step (b) ofpatterning the conductive layer into a specified pattern to form ascanning layer having a first electrode; step (c) of forming aninsulation layer entirely on the substrate to cover the scanning line;step (d) of forming a hole reaching the first electrode at a specifiedposition of the insulation layer in a specified shape by aphotolithographic method; step (e) of forming a zinc sulfide layer onthe first electrode in the hole, the zinc sulfide layer having such aspecified thickness as to have an I-V characteristic expressed by acontinuous curve; and step (f) of forming a pixel electrode on theinsulation layer in such a pattern as to be electrically connected tothe zinc sulfide layer. The two-terminal element includes the firstelectrode, the zinc sulfide layer and a part of the pixel electrodelocated on an area corresponding to the zinc sulfide layer.

In still another aspect of the invention, a method for producing adisplay apparatus including a two-terminal element includes step (a) offorming a first conductive layer on an insulating substrate; step (b) ofpatterning the first conductive layer into a specified pattern to form ascanning layer having a first electrode; step (c) of forming aninsulation layer entirely on the substrate to cover the scanning line;step (d) of forming a photoresist on the insulation layer and performingexposure and development using a mask for regulating a position and ashape for the formation of a zinc sulfide layer; step (e) of etching theinsulation layer using the photoresist as a mask in order to form a holereaching the first electrode in the insulation layer; step (f) offorming the zinc sulfide layer on the first electrode in the hole, thezinc sulfide layer having such a specified thickness as to have an I-Vcharacteristic expressed by a continuous curve; step (g) of forming asecond conductive layer on the zinc sulfide layer; step (h) of removingthe photoresist, a part of the zinc sulfide layer corresponding to thephotoresist, and a part of the second conductive layer corresponding tothe photoresist by a lift-off method so as to keep the zinc sulfidelayer at the position regulated by the mask and to form a secondelectrode from the second conductive layer; and step (i) of forming apixel electrode on the insulation layer in such a specified pattern asto be electrically connected to the second electrode. The two-terminalelement includes the first electrode, the zinc sulfide layer and thesecond electrode.

In still another aspect of the invention, a method for producing adisplay apparatus including a two-terminal element includes step (a) offorming a conductive layer on an insulating substrate; step (b) ofpatterning the conductive layer into a specified pattern to form ascanning layer having a first electrode; step (c) of forming a zincsulfide layer entirely on the substrate to cover the scanning line, thezinc sulfide layer having such a thickness as to have an I-Vcharacteristic expressed by a continuous curve; step (d) of forming aphotoresist on the zinc sulfide layer and removing a part of the zincsulfide layer and a part of the photoresist corresponding to an areaother than a specified area on the first electrode; step (e) of formingan insulation layer entirely on the substrate to cover the photoresistand the zinc sulfide layer; step (f) of removing the photoresist and apart of the insulation layer on the photoresist by a lift-off method;and step (g) of forming a pixel electrode on the insulation layer insuch a specified pattern as to be electrically connected to the zincsulfide layer. The two-terminal element includes the first electrode,the zinc sulfide layer and a part of the pixel electrode on thespecified area.

In still another aspect of the invention, a method for producing adisplay apparatus including a two-terminal element includes step (a) offorming a first conductive layer on an insulating substrate; step (b) ofpatterning the first conductive layer into a specified pattern to form ascanning layer having a first electrode; step (c) of forming a zincsulfide layer entirely on the substrate to cover the scanning line, thezinc sulfide layer having such a thickness as to have an I-Vcharacteristic expressed by a continuous curve; step (d) of forming asecond conductive layer on the zinc sulfide layer; step (e) of forming aphotoresist on the second conductive layer and performing exposure anddevelopment using a mask for regulating a position and a shape for theformation of a zinc sulfide layer; step (f) of removing the photoresist,a part of the zinc sulfide layer corresponding to the photoresist, and apart of the second conductive layer corresponding to the photoresist bya lift-off method to keep the zinc sulfide layer at the positionregulated by the mask and to form a second electrode from the secondconductive layer; and step (g) of forming an insulation layer entirelyon the substrate so as to cover the photoresist and the zinc sulfidelayer; step (h) of removing the photoresist and a part of the insulationlayer on the photoresist by a lift-off method; and step (i) of forming apixel electrode on the insulation layer in such a specified pattern asto be electrically connected to the second electrode. The two-terminalelement includes the first electrode, the zinc sulfide layer and thesecond electrode.

Thus, the invention described herein makes possible the advantages of(1) providing a display apparatus having an excellent I-Vcharacteristic, the few defects in pixels, and high uniformity of thedisplay quality; and (2) a highly reliable display apparatus and amethod for producing the same which solves the problem of insulationbreakdown due to insufficient insulation of a tapered side portion of afirst metal layer by an active layer.

These and other advantages of the present invention will become apparentto those skilled in the art upon reading and understanding the followingdetailed description with reference to the accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partial top view of an LCD apparatus in a first exampleaccording to the present invention;

FIG. 2 is a cross sectional view of the display apparatus illustrated inFIG. 1 looking along section line 2'--2' in FIG. 1;

FIG. 3 is an isometric view of the display apparatus shown in FIGS. 1and 2;

FIG. 4 is a graph illustrating a curve representing the I-Vcharacteristic of a two-terminal device used in the LCD apparatus in thefirst example;

FIG. 5 is an equivalent circuit diagram corresponding to one pixel ofthe LCD apparatus in the first example;

FIG. 6 is a partial top view of an LCD apparatus in a second exampleaccording to the present invention;

FIG. 7 is a cross sectional view of the LCD apparatus shown in FIG. 6looking along section line 7'--7' in FIG. 6;

FIG. 8 is a graph illustrating curves representing the I-Vcharacteristic of the zinc sulfide (ZnS) layers including aluminum (Al)in various weight ratios in an LCD apparatus in a fourth exampleaccording to the present invention;

FIG. 9 is a graph illustrating curves representing the I-Vcharacteristic of the ZnS layers with various composition ratios of Znand S in an LCD apparatus in a fifth example according to the presentinvention;

FIG. 10 is a cross sectional view of an LCD apparatus in a sixth exampleaccording to the present invention;

FIG. 11 is a cross sectional view of a modification of the LCD apparatusin the sixth example;

FIG. 12 is a cross sectional view of another modification of the LCDapparatus in the sixth example;

FIG. 13 is a graph illustrating a curve representing the Q-Vcharacteristic of the LCD apparatus shown in FIG. 12;

FIG. 14 is a partial cross sectional view of an LCD apparatus in a ninthexample according to the present invention;

FIG. 15 is a partial cross sectional view of a modification of the LCDapparatus in the ninth example;

FIG. 16(a)-16(f) is a schematic illustration of a part of a process forproducing the LCD apparatus illustrated in FIG. 14;

FIG. 17 is a top view of a modification of the LCD apparatus describedin any one of the first through the ninth examples;

FIG. 18 is a partial top view of an LCD apparatus in a tenth and aneleventh examples according to the present invention;

FIG. 19 is a cross sectional view of the LCD apparatus shown in FIG. 18looking across section line 19'--19';

FIG. 20(a)-20(g) is a schematic illustration of a part of a process forproducing the LCD apparatus shown in FIG. 18 and 19 in the tenthexample;

FIG. 21(a)-21(g) is a schematic illustration of a part of a process forproducing the LCD apparatus shown in FIGS. 18 and 19 in the eleventhexample;

FIG 22 is a partial cross sectional view of an LCD apparatus in atwelfth and a thirteenth examples according to the present invention;

FIG. 23(a)-23(h) is a schematic illustration of a part of a process forproducing the LCD apparatus shown in FIG. 22 in the twelfth example;

FIG. 24(a)-24(h) a schematic illustration of a part of a process forproducing the LCD apparatus shown in FIG. 22 in the thirteenth example;

FIG. 25 is a partial cross sectional view of an LCD apparatus in afourteenth and a fifteenth examples according to the present invention;

FIG. 26 is a partial top view of a conventional LCD apparatus;

FIG. 27 is a cross sectional view of the LCD apparatus illustrated inFIG. 26 looking along section line 27'--27' in FIG. 26;

FIG. 28 is a graph illustrating a curve representing the I-Vcharacteristic of a conventional MIM element;

FIG. 29 is a cross sectional view illustrating another conventional MIMelement;

FIG. 30(a)-30(f) is a schematic illustration of a process for producinga conventional MIM element used in a conventional display apparatus; and

FIG. 31 is a graph illustrating a curve representing the I-Vcharacteristic of ZnS film used in a conventional display apparatus.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A display apparatus according to the present invention includes atwo-terminal device having an active layer formed of a thin zinc sulfide(ZnS) film. The nonlinearity in the electric resistance of the active isutilized for display. It is known that there are mainly two types of I-Vcharacteristic: the Poole-Frenkcl characteristic and the switchingcharacteristic. The switching characteristic indicates a high resistancestate and a low resistance state. The Poole-Frenkcl characteristic isdescribed in H. Murry, Thin Solid Films 22 (1974) 37, and the switchingcharacteristic is described in Masahiro Fukuzawa, Applied Physics Vol.46, No. 7 (1977).

FIG. 31 is a graph illustrating a curve representing the I-Vcharacteristic of the ZnS film which indicates the switchingcharacteristic. As is apparent from FIG. 31, the resistance of the ZnSfilm drastically changes at a threshold level of the applied voltageV_(TH), and thus the curve representing the I-V characteristic isdiscontinuous. As applied voltage is increased, at the threshold levelof the voltage V_(TH), the current begins to flow and the voltagedecreases to a certain level (V_(O)). And then the current starts toincrease proportionally to the applied voltage. The curve has slopes inopposite inclination. The I-V characteristic curve has different valuesof current I corresponding to the same value of voltage V between V_(O)and V_(TH). The I-V characteristic of the ZnS around the threshold levelof the applied voltage is extremely unstable and is not reliable. A filmhaving such an I-V characteristic is not suitable for a displayapparatus.

By controlling a condition of the ZnS film, an I-V characteristicshowing a continuous curve can be obtained. In this specification,continuous means that the curve has a continuous slope in which theinclination increases or decreases but does not change to the oppositedirection.

FIG. 4 shows a curve representing the I-V characteristic of a ZnS filmused in a display apparatus according to the present invention. In thepresent invention, a ZnS thin film which realizes an I-V characteristicshowing a continuous curve is used, and therefore the I-V characteristicis stable even around the threshold level of the applied voltage.Moreover, the I-V characteristic of the ZnS film is better in steepnessthan that of the insulation layer (active layer) formed of Ta₂ O₅ orSiN_(x). This means that the ZnS film used in the present invention issuitable for the switching device.

In FIG. 4, parts of the curve representing the I-V characteristic in twoquadrants corresponding to the positive valves of the voltage V and thenegative values of the voltage V are highly symmetrical to each other.Such symmetry is important for realizing high quality images in anAC-driven LCD apparatus.

Hereinafter, the present invention will be described by way ofillustrative examples with reference to the accompanying drawings.

EXAMPLE 1

A first example of a display apparatus according to the presentinvention will be described with reference to FIGS. 1 through 5.

FIGS. 1 and 2 show a part of an LCD apparatus 20 in the first exampleaccording to the present invention. FIG. 1 is a top view of arectangular pixel electrode 5 and a vicinity thereof in the LCDapparatus 20; and FIG. 2 is a cross sectional view of the LCD apparatus20 shown in FIG. 1 looking along section line 2'--2' in FIG. 1.

As is shown in FIG. 1, the LCD apparatus 20 includes an insulating basesubstrate 1 formed of glass or the like. A plurality of pixel electrode5 (only one is shown in FIG 1) are arranged in a matrix on a top surfaceof the base substrate 1. A plurality of scanning lines 2 (only one isshown in FIG. 1) respectively corresponding to the pixel electrodes 5are also on the top surface of the base substrate 1, each for sending atiming signal to the corresponding pixel electrode 5 in order to drivethe pixel electrode 5. Each pixel electrode 5 and the correspondingscanning line 2 are connected to each other by a switching element.According to the present invention, a two-terminal device including aZnS layer is used as the switching device. The continuous and steep I-Vcharacteristic of the ZnS layer is used.

A first electrode 4 is branched from the scanning line 2. The firstelectrode 4 includes a first section 6 extended perpendicularly to thescanning line 2 and a second section 7 extended perpendicularly to thefirst section 6. The second section 7 is parallel to the scanning line2. A ZnS layer 3 formed of a thin ZnS film is on the entire top surfaceof the base substrate 1, covering the scanning line 2 and the firstelectrode 4. The pixel electrode 5 is on the ZnS layer 3 along thescanning line 2. As is illustrated in FIG. 2, a second electrode 8 isbranched from the pixel electrode 5 in such a direction as to cross thesecond section 7 of the first electrode 4. The second electrode 8partially covers the second section 7 of the first electrode 4 with theZnS layer sandwiched therebetween. A two-terminal device 14 includes athree-layer structure including the first electrode 4, the ZnS layer 3and the second electrode 8. An alignment film 9 is on the entire basesubstrate 1, covering the ZnS layer 3 and the pixel electrode 5.

FIG. 3 is a partial isometric view of the display apparatus 20 shown inFIGS. 1 and 2. As is shown in FIG. 3, an insulating counter substrate 12formed of glass or the like is opposed to the base substrate 1 with aliquid crystal layer 10 as a display medium sandwiched therebetween. Aplurality of counter electrodes 13 are on the bottom surface of thecounter substrate 12 facing the base substrate 1, the counter electrodes13 being arranged in such a direction as to cross the scanning line 2.An alignment film 11 is on the entire bottom surface of the countersubstrate 12, covering the counter electrodes 13.

The LCD apparatus 20 having the above-described structure is produced inthe following manner.

On the insulating base substrate 1 formed of glass or the like, aconductive thin film formed of Ta is formed in a specified thickness(300 nm in the first example) by sputtering. The conductive thin filmmay be formed by CVD, evaporation or the like instead of sputtering.

As shown in FIG. 2, the conductive thin film formed of Ta is patternedinto a specified pattern so as to form the scanning line 2 having thefirst electrode 4. The ZnS layer 3 is formed on the entire top surfaceof the base substrate 1 in a specified thickness by sputtering so as tocover the scanning line 2 and the first electrode 4. The thickness ofthe ZnS layer 3 is determined as follows based on the driving voltagefor the LCD apparatus 20. Namely, the voltage which is necessary tocause liquid crystal molecules in the liquid crystal layer 10 as thedisplay medium to display an image.

When an electric field applied to the ZnS layer 3 reaches a certainlevel, the current suddenly flows in the ZnS layer 3. Such a specificlevel is referred to as the threshold level of the electric field (orwithstand voltage). The threshold voltage depends on the thickness ofthe ZnS layer 3. Accordingly, the thickness of the ZnS layer 3 isdetermined so that the voltage applied to the liquid crystal layer 10will reach the driving voltage for display when the voltage applied tothe ZnS layer 3 exceeds the threshold level. Namely, when the ZnS layer3 as the switching device is turned "ON".

The threshold level of the electric field is in proportion to thethickness of the ZnS layer 3. Where the thickness is less than 10 nm,the I-V characteristic is unstable; and where the thickness is more than1 μm, the steepness of the I-V characteristic declines. Accordingly, inorder to obtain a stable and steep I-V characteristic, the thickness ofthe ZnS layer 3 should be between 10 nm and 1 μm, preferably 30 nm to200 nm inclusive.

As the sputtering target, a highly pure, sintered ZnS target is used.Or, highly pure ZnS powders which are densely spread all over a quartzglass plate may be used. The sputtering is performed using argon (Ar) asthe sputtering gas, at the substrate temperature of 250° C., with thegas pressure of 10 Pa and the input power of 750 W.

The ZnS layer 3 may be formed by CVD, evaporation or other thin filmformation methods instead of sputtering. In the first example, thethickness of the ZnS layer 3 is 100 nm.

A conductive film for forming the pixel electrode 5 is formed on the ZnSlayer 3. In the first example, an ITO film is formed by sputtering asthe conductive film since ITO is suitable for a transmission type LCDapparatus due to the transparency thereof. The ITO film may be formed byCVD, evaporation or the like instead of sputtering.

For a reflection type LCD apparatus, the conductive film may be formedof metal such as Al, Ni, Ti, Ag, Cr, Cu and alloys thereof.

The ITO film is patterned into a specified pattern in order to form thepixel electrode 5 and the second electrode 8 which is extended in such adirection as to cross the second section 7 of the first electrode 4 ofthe scanning line 2.

The alignment film 9 is formed on the entire base substrate 1. Indetail, the material for the alignment film 9 containing, for example,polyimide is formed on the entire base substrate 1, and then thematerial is cured and aligned.

As shown in FIG. 3, on the bottom surface of the counter substrate 12 toface the base substrate 1, an ITO film is formed and patterned so as toform a plurality of counter electrodes 13 corresponding to the pluralityof pixel electrodes 5, respectively. The counter electrodes 13 arearranged substantially in parallel to one another. Each counterelectrode 13 has a width sufficient to cover the corresponding pixelelectrode 5. Then, the alignment film 11 is formed on the entire bottomsurface of the counter substrate 12. In detail, a material for thealignment film 11 is formed on the entire bottom surface of the countersubstrate 12 so as to cover the counter electrodes 13. The material iscured and then aligned.

The base substrate 1 having the scanning lines 2, the pixel electrodes 5and the like on the top surface thereof and the counter substrate 12having the counter electrodes 13 and the like on the bottom surfacethereof are assembled with spacers (not shown) sandwiched therebetweenso that the top surface of the base substrate 1 and the bottom surfaceof the counter substrate 12 face each other and so that the scanninglines 2 and the counter electrodes 13 are arranged in such directions asto cross each other.

The liquid crystal is injected between the base substrate 1 and thecounter substrate 12 to form the liquid crystal layer 10, and then theliquid crystal layer 10 is sealed.

The LCD apparatus 20 produced in the above-described manner operates inthe following way.

As is mentioned above, FIG. 4 is a graph illustrating the curverepresenting the I-V characteristic of a two-terminal device used in thepresent invention, for example, the two-terminal device 14. The currentI is represented in logarithmic scale in FIG. 4. FIG. 5 is an equivalentcircuit diagram corresponding to one pixel of an LCD apparatus includingthe two-terminal device, for example, the LCD apparatus 20.

As is shown in FIG. 5, the two-terminal device 14 has a capacitanceC_(D), and the liquid crystal layer has a capacitance C_(L). The pixelis equivalent to a circuit including the capacitances C_(D) and C_(L)connected with each other in series between the first electrode 4 andthe counter electrode 13.

When a voltage is applied to two ends of the LCD apparatus 20, namely,to the first electrode 4 and to the counter electrode 13, a voltageV_(D) applied to the two-terminal device 14 is expressed by Equation(4). In other words, the voltage V_(D) is obtained bycapacitance-dividing of the voltage V.

    V.sub.D =V·C.sub.L /(C.sub.L +C.sub.D)            (4)

A voltage V_(L) applied to the capacitance of the liquid crystal layer10 is expressed by Equation (5).

    V.sub.L =V·C.sub.D /(C.sub.L +C.sub.D)            (5)

It is apparent from Equation (4) that, when C_(L) >>C_(D), V_(D) isapproximately equal to V. Accordingly, the voltage V_(D) at a sufficientlevel is applied to the two-terminal device 14. When the voltage V_(D)applied to the two-terminal device 14 exceeds the threshold voltageV_(TH) of the two-terminal device 14, the two-terminal device 14 isturned ON.

When the two-terminal device 14 is ON, the voltage V_(L) expressed byEquation (5) is applied to the liquid crystal layer 10, and thus anelectrical charge is stored in the capacitance C_(L) of the liquidcrystal layer 10.

When the voltage V_(D) applied to the two-terminal device 14 isdecreased to below the threshold voltage V_(TH) of the two-terminaldevice 14, the two-terminal device 14 is turned OFF. Even when thetwo-terminal device 14 is OFF, the liquid crystal molecules are drivenby the electric charge stored in the capacitance C_(L) of the liquidcrystal layer 10 when the two-terminal device 14 is ON.

EXAMPLE 2

Referring to FIGS. 6 and 7, a second example of a display apparatusaccording to the present invention will be described. In this and allthe following examples, identical elements as those in the first examplebear identical reference numerals therewith for simplicity.

FIGS. 6 and 7 show a part of an LCD apparatus 30 in the second exampleaccording to the present invention. FIG. 6 is a top view of arectangular pixel electrode 5 and a vicinity thereof in the LCDapparatus 30; and FIG. 7 is a cross sectional view of the LCD apparatus30 shown in FIG. 6 looking along section line 7'--7' in FIG. 6.

The structure of the LCD apparatus 30 in the second example is identicalwith the structure of the LCD apparatus 20 in the first example exceptthat a second electrode 15 formed of metal is provided independentlyinstead of being extended from the pixel electrode 5.

The second electrode 15 is on the second section 7 of the firstelectrode 4 with the ZnS layer 3 sandwiched therebetween. The pixelelectrode 5 is on the ZnS layer 3, covering an end portion of the secondelectrode 15. The two-terminal device 14 includes the three-layerstructure including the first electrode 4, the ZnS layer 3 and thesecond electrode 15. Due to such a structure, the ZnS layer 3 and thesecond electrode 15 are electrically connected to each other.

In the second example, the process for producing the LCD apparatus 30requires the step of forming the second electrode 15 in addition to theproduction process of the LCD apparatus 20 described in the firstexample. However, since the second electrode 15 and the pixel electrode5 are formed of different materials from each other, the materials forthe second electrode 15 and the pixel electrode 5 can be chosen from awider selection of materials than in the first example.

EXAMPLE 3

In a third example according to the present invention, the ZnS layer 3is formed by sputtering using a different target from the target used inthe first and the second examples. The target used in the third exampleis obtained by doping manganese (Mn) in the sintered ZnS target. Thetarget may be obtained by mixing Mn in highly pure ZnS powders and thendensely spreading the resultant mixture all over a quartz glass plate.In this manner, Mn is implanted into the ZnS layer 3.

Implantation of Mn into the ZnS layer 3 improves the withstand voltageof the ZnS layer 3 and thus enhances the reliability of the two-terminaldevice 14. Except for the material for the sputtering target, the LCDapparatus in the third example is identical with the LCD apparatuses inthe first and the second examples.

Instead of Mn, other materials may be used. For example, copper, rareearth elements such as terbium (Tb), samarium (Sm), europium (Eu), oroxides, fluorides, carbonates, phosphates, oxalates, chlorides,bromides, iodides, nitrates or other compounds of such rare earthelements may be used. By mixing powders of these elements or compoundswith the ZnS powders, at least one of these elements or compounds isimplanted into the ZnS layer 3. In this manner, the withstand voltage ofthe ZnS layer 3 is improved, and thus the reliability of thetwo-terminal device 14 is enhanced.

The two-terminal device in the third example has an I-V characteristicexcellent in steepness and symmetry.

EXAMPLE 4

In a fourth example according to the present invention, the ZnS layer 3is formed by sputtering using a different target from the target used inthe previous three examples. The target used in the fourth example isobtained by doping Al in the sintered ZnS target. The target may beobtained by mixing Al powders in highly pure ZnS powders and thendensely spreading the resultant mixture all over a quartz glass plate.In this manner, Al is implanted into the ZnS layer 3.

FIG. 8 illustrates curves representing the I-V characteristic of the ZnSlayers 3 including Al in various weight ratios. As is apparent from FIG.8, the I-V characteristic can be changed by the amount of Al included inthe ZnS layer 3. As the amount of Al increases, the I-V characteristicincreases in steepness.

Except for the material for the sputtering target, the LCD apparatus inthe fourth example is identical with the LCD apparatuses in the previousthree examples.

Instead of Al, other III-group elements may be included in the ZnS layer3 by implantation into the ZnS powders and the same effects can beobtained.

As is mentioned above, the I-V characteristic of the ZnS layer 3 caneasily be changed by varying the amount of Al or other III-groupelements included in the ZnS layer 3. Utilizing this fact, atwo-terminal device having an optimum I-V characteristic can be obtainedin accordance with the electrooptic properties of the display mediumused in an LCD apparatus to be produced.

EXAMPLE 5

In a fifth example according to the present invention, the ZnS layer 3is formed by sputtering using a different target from the target used inthe previous four examples. The target used in the fifth example isobtained by doping Zn in the sintered ZnS target. In this manner, thecomposition ratio of Zn and sulfur (S) in the ZnS layer 3 can bechanged.

FIG. 9 illustrates curves representing the I-V characteristic of the ZnSlayers 3 including Zn in various composition ratios with respect to S.As is apparent from FIG. 9, the I-V characteristic can be changed byvarying the composition ratio of Zn to S in the ZnS layer 3. As thecomposition ratio of Zn increases, the I-V characteristic increases insteepness.

Except for the material for the sputtering target, the LCD apparatus inthe fifth example is identical with the LCD apparatuses in the previousfour examples.

As is mentioned above, the I-V characteristic of the ZnS layer 3 caneasily be changed by varying the composition ratio of Zn to S includedin the ZnS layer 3. Utilizing this fact, a two-terminal device having anoptimum I-V characteristic can be obtained in accordance with theelectrooptic properties of the display medium used in an LCD apparatusto be produced.

EXAMPLE 6

Referring to FIGS. 10 through 13, a sixth example of a display apparatusaccording to the present invention will be described.

FIG. 10 is a cross sectional view of an LCD apparatus 31 in the sixthexample according to the present invention. In the two-terminal device14 in the LCD apparatus 31, an insulation layer 21 is sandwiched betweenthe ZnS layer 3 (active layer) and the second electrode 8 of the pixelelectrode 5. The insulation layer 21 is formed of SiN_(x) by sputteringon the entire base substrate 1. The pixel electrode 5 having the secondelectrode 8 is formed on the insulation layer 21.

By such a structure, the withstand voltage of the two-terminal device 14is improved, and thus the reliability of the two-terminal device 14 isenhanced.

The insulation layer 21 may be formed of any insulating material,preferably a nitrogen compound such as SiN_(x) or AlN.

The production process of the LCD apparatus 31 is identical with theproduction process described in the first example until the ZnS layer 3is formed. After the formation of the ZnS layer 3, the insulation layer21 is formed on the entire ZnS layer 3 by sputtering, using silicon as asputtering target under an atmosphere of a mixture gas of Ar and N₂, atthe substrate temperature of 250° C., the gas pressure of 8 Pa and theinput power of 750 W.

On the insulation layer 21, the pixel electrode 5 is formed. After that,the production process of the LCD apparatus 31 is again identical withthe process described in the first example.

FIG. 11 is a cross sectional view of an LCD apparatus 32 as amodification of the LCD apparatus 31. In the LCD apparatus 32, theinsulation layer 21 is sandwiched between the first electrode 4 and theZnS layer 3.

FIG. 12 is a cross sectional view of an LCD apparatus 33 as anothermodification of the LCD apparatus 31. In the LCD apparatus 33, aninsulation layer 21 is sandwiched between the first electrode 4 and theZnS layer 3, and another insulation layer 21 is sandwiched between theZnS layer 3 and the second electrode 8.

The LCD apparatus 33 including the two-terminal device 14 having twoinsulation layers 21 is driven, utilizing the nonlinearity of thecapacitance of the two-terminal device 14. FIG. 13 is a graphillustrating the curve representing the Q-V characteristic of thetwo-terminal device 14 of the LCD apparatus 33. As is apparent from FIG.13, the Q-V characteristic is satisfactory both in steepness andsymmetry.

As is mentioned above, the insulation layer 21 may be formed of anyinsulating material. In the LCD apparatus 33, the two insulation layers21 are preferably formed of an identical material in order to obtain asatisfactory Q-V characteristic.

The insulation layer 21 may have a multiple layer structure instead of asingle layer structure. For example, the insulation layer 21 may have atwo-layer structure of SiO₂ and SiN_(x) or have a three-layer structureof SiO₂, Ta₂ O₅ and SiN_(x). In the LCD apparatus 33, either one of thetwo insulation layers 21 sandwiching the ZnS layer 3 may include such amultiple layer structure. The two insulation layers 21 may both includea multiple layer structure. By such a structure, the withstand of thetwo-terminal device 14 is improved, and thus the reliability of thetwo-terminal device 14 is enhanced.

The ZnS layer 3 and the insulation layer 21 both transmit visible light.Therefore, there is no need to pattern the ZnS layer 3 and theinsulation layer 21 to remove any part thereof corresponding to adisplay area (namely, an area except for the two-terminal device 14).Accordingly, the formation of the layers can be performed continuouslyby a thin film formation technology after the first electrode 4 isformed. Such continuous formation simplifies the production process andthus reduces the production cost.

EXAMPLE 7

In a seventh example according to the present invention, the ZnS layer 3described in the first through sixth examples illustrated in, forexample, FIGS. 2, 7 and 10 through 12 is heated. The heating temperatureshould be equal to or higher than the substrate temperature used for theformation of ZnS layer 3. The heating is performed in a vacuum oven, inthe temperature range of 300° C. to 600° C. inclusive for 1 hour.Instead of the vacuum oven, an ordinary oven or a sulfur atmosphere maybe used.

The ZnS layer 3 illustrated in FIGS. 2, 7 and 11 is heated before theformation of the pixel electrode 5 having the second electrode 8 or 15.The ZnS layer 3 may be heated after the formation of the pixel electrode5 having the second electrode 8 or 15.

The ZnS layer 3 illustrated in FIG. 10 is heated before the insulationlayer 21 is formed or after the pixel electrode 5 including the secondelectrode 8 is formed. The ZnS layer 3 illustrated in FIG. 12 is heatedbefore the formation of the insulation layer 21 on the ZnS layer 3 orafter the formation of the pixel electrode 5 having the second electrode8.

By heating the ZnS layer 3, the crystallinity of the ZnS layer 3 isimproved. Further, the elements and the compounds described in thethird, fourth and fifth examples are dispersed into the ZnS layer 3 bysuch heating when these elements and the compounds are implanted intothe ZnS layer 3, and thus the ZnS layer 3 is made uniform in quality.Accordingly, the uniformity of characteristics and the reliability ofthe two-terminal device 14 are enhanced.

An aging test of the two-terminal device 14 produced according to theproduction process described in the seventh example proves that thetwo-terminal device 14 in the seventh example has stable characteristicswith less aging. An LCD apparatus including such a two-terminal device14 has stable display characteristics.

EXAMPLE 8

In an eighth example according to the present invention, a two-terminaldevice 14 is produced in accordance with the production processdescribed in the sixth example, and an LCD apparatus is also producedusing such a two-terminal device 14.

As is shown in FIG. 10, the insulation layer 21 is formed on the ZnSlayer 3, and the pixel electrode 5 having the second electrode 8 isformed on the insulation layer 21. The pixel electrode 5 is formed of atransparent conductive material such as ITO, and the insulation layer 21is formed of a nitrogen compound.

As is mentioned in the seventh example, heating the ZnS layer 3 improvesthe characteristics of the two-terminal device 14 as the switchingdevice. However, in the case that the pixel electrode 5 is formed of ITOand the insulation layer 21 is formed of an oxide such as Ta₂ O₅ or Y₂O₃, heating the ZnS layer 3 at a high temperature oxidizes a part of thepixel electrode 5 in contact with the insulation layer 21 and thusincreases the surface resistance of the pixel electrode 5 formed of ITO.In the eighth example, the insulation layer 21 is formed of a nitrogencompound in order to solve such an inconvenience. An LCD apparatusincluding such a two-terminal device 14 is produced in the same manneras that described in the first example.

It has been confirmed that the use of silicon oxide for the insulationlayer 21 keeps the surface resistance of the pixel electrode 5 smalleven if the ZnS layer 3 is heated.

An aging test of the two-terminal device 14 formed in the eighth exampleproves that the two-terminal device 14 has stable characteristics withless aging as in the seventh example. The LCD apparatus including such atwo-terminal device 14 also has stable display characteristics.

EXAMPLE 9

Referring to FIGS. 14 through 17, a ninth example of a display apparatusaccording to the present invention will be described. FIG. 14 is a crosssectional view of an LCD apparatus in a ninth example according to thepresent invention.

As is illustrated in FIG. 14, in an LCD apparatus 34 in the ninthexample, an insulation layer 22 is sandwiched between the ZnS layer 3and the pixel electrode 5 having the second electrode 8. The insulationlayer 22 is formed by coating the ZnS layer 3 with a polymeric resinbefore forming the pixel electrode 5 having the second electrode 8. Theinsulation layer 22 also acts as a protection layer for the ZnS layer 3.The insulation layer 22 has a hole 23, and the hole 23 is filled withthe second electrode 8. The ZnS layer 3 and the second electrode 8 areelectrically connected with each other through the hole 23. Except forthe insulation layer 22, the LCD apparatus 34 is identical in structurewith the LCD apparatuses described in the first through the fifthexamples.

The LCD apparatus 34 shown in FIG. 14 is produced in the followingmanner.

The production process of the LCD apparatus 34 is identical with theproduction process described in the first example until the ZnS layer 3is formed. After the formation of the ZnS layer 3, the ZnS layer 3 iscoated with a polymeric resin using a spinner, a roller coater or thelike to form the insulation layer 22. The insulation layer 22 formed ofa polymeric resin protects the ZnS layer 3 without influencing electriccharacteristics of the two-terminal device 14 of the LCD apparatus 34.Inorganic substances such as SiO₂ or Al₂ O₃ and organic substances suchas acrylic resins, polyimide or polyurea may be used for a protectionlayer for the ZnS layer 3 instead of a polymeric resin.

After the formation of the insulation layer 22, the hole 23 is formed inthe insulation layer 22, at a portion thereof corresponding to the firstelectrode 4 by a photographic method. Then, the pixel electrode 5 havingthe second electrode 8 is formed in such a pattern that the secondelectrode 8 fills the hole 23. As is mentioned above, the ZnS layer 3and the second electrode 8 (or the pixel electrode 5) are electricallyconnected with each other through the hole 23.

FIG. 16 is a schematic illustration of the process for forming the hole23.

After the insulation layer 22 is formed on the ZnS layer 3 (part (a)),the insulation layer 22 is coated with a negative photoresist 24, andthe resultant lamination is exposed to light from the side of a bottomsurface of the base substrate 1, using the first electrode 4 as a mask(part (b)). Then, a part of the negative photoresist 24 shielded fromthe light by the first electrode 4 is removed by development andpatterning (part (c)). A part of the insulation layer 22 correspondingto the first electrode 4 is removed in accordance with the pattern ofthe photoresist 24 to form the hole 23 (part (d)). Then, the photoresist24 is peeled off (part (e)), and the pixel electrode 5 having the secondelectrode 8 is formed on the insulation layer 22 to fill the hole 23(part (f)). The pixel electrode 5 is electrically connected to the ZnSlayer 3 through the hole 23.

According to such a method, the insulation layer 22 is patterned to formthe hole 23 in a self-alignment manner. Therefore, the hole 23 is formedat an appropriate position with no positional displacement. Theproduction process is simple. As a result, a highly reliabletwo-terminal device 14 is produced with high efficiency.

An aging test of the two-terminal device 14 produced in the ninthexample proves that the two-terminal device 14 has stablecharacteristics with little aging as in the seventh and the eighthexamples. This is due to the insulation layer 22 formed of a polymericresin. The LCD apparatus 34 including such a two-terminal device 14 alsohas stable characteristics.

In the first through the ninth examples, the first electrode 4 isbranched from the scannng line 2. Instead, a specified area within thescanning line 2 may be designated as the first electrode 4 as isillustrated in FIG. 17. In such a case, the two-terminal device 14 isformed by laminating the ZnS layer 3 (active layer) and the secondelectrode 8 on the first electrode 4 at the specified area within thescanning line 2.

FIG. 15 is a cross sectional view of a modification of the LCD apparatus34 in the ninth example. In an LCD apparatus 35 as the modification, theinsulation layer 21 formed of SiN_(x) or the like is sandwiched betweenthe second electrode 4 and the ZnS layer 3 in addition to the insulationlayer 22.

EXAMPLE 10

In a tenth example according to the present invention, a two-terminaldevice for a display apparatus and a method for producing the same forsolving the problem of insufficient insulation of the first electrode 4by the ZnS layer 3 will be described with reference to FIGS. 18 through20.

FIG. 18 is a top view of two pixels and the vicinity thereof in an LCDapparatus 36 in the tenth example; and FIG. 19 is a cross sectional viewof the LCD apparatus shown in FIG. 18 looking across section line19'--19' in FIG. 18.

As is shown in FIG. 18, the LCD apparatus 36 includes the basesubstrate 1. The scanning line 2 is on the top surface of the basesubstrate 1, and the first electrode 4 is branched from the scanningline 2. The ZnS layer 3 acting as an active layer is on a specified areaof the first electrode 4. It should be noted that the ZnS layer 3 is noton the entire base substrate 1. As is shown in FIG. 19, in a hole 19, aninsulation layer 16 is on the base substrate 1, covering the firstelectrode 4 except for an the area on which the ZnS layer 3 is located.The insulation layer 16 has a greater thickness than that of the ZnSlayer 3. The pixel electrode 5 is on the insulation layer 16, coveringthe ZnS layer 3. A part of the pixel electrode 5 which is on the ZnSlayer 3 is the second electrode 8. The two-terminal device 14 includesthe three-layer structure including the first electrode 4, the ZnS layer3 and the second electrode 8.

The LCD apparatus 36 having the above-described structure is produced inthe following manner.

FIG. 20 is a schematic illustration of a part of the process forproducing the LCD apparatus 36.

First, a film of Ta is formed on the base substrate 1 in a specifiedthickness by sputtering and patterned into a specified pattern to formthe scanning line 2 having the first electrode 4 (part (a)). Thethickness of the scanning line 2 is 300 nm in the tenth example. Thescanning line 2 having the first electrode 4 may be formed of aconductive thin film of titanium, molybdenum, aluminum, copper, ITO orthe like instead of Ta. As an etchant for patterning, a mixed solutionof hydrofluoric acid and nitric acid is used.

Then, the insulation layer 16 is formed as is illustrated in parts (b)through (d).

A film of SiO_(x) is formed on the base substrate 1 to cover the firstelectrode 4 in a thickness of 1.0 μm (part (b)). Then, the film ofSiO_(x) is patterned into a specified pattern using a photoresist 17 anda mask 18 (parts (c) and (d)). In this way, the insulation layer 16having a hole 19 at a portion corresponding to the two-terminal device14 is formed. As the photoresist 17, a positive photoresist OFPR-800produced by Tokyo Ohka Kabushiki Kaisha is used in the tenth example,but a negative photoresist may be used. The patterning after developmentis performed by dry etching. The insulation layer 16 may be formed of aninsulating material such as SiN_(x), Al₂ O₃, are AlN instead of SiO_(x).

Then, the ZnS layer 3 is formed on the photoresist 17 to cover the firstelectrode 4 by sputtering in a thickness of 200 nm (part (e)).

The photoresist 17 and the ZnS layer 3 on the photoresist 17 are peeledoff by a lift-off method (part (f)) to keep the ZnS layer 3 only on thefirst electrode 4 in the hole 19. As the peeling-off liquid, OMR-710produced by Tokyo Ohka Kabushiki Kaisha is used.

A film of ITO is formed on the insulation layer 16 to cover the ZnSlayer 3 to a thickness of 200 nm, and patterned to form the pixelelectrode 5 having the second electrode 8 (part (g)). The pixelelectrode 5 having the second electrode 8 may be formed of a conductivematerial such as aluminum, tantalum, titanium, molybdenum or copperinstead of ITO.

The two-terminal device 14 produced in the above-described mannerincludes a flat portion of the first electrode 4 but excludes a sideportion and the vicinity thereof including an edge formed at thejunction of a top surface 28 and a side surface 29 of the firstelectrode 4 and another edge formed at the junction of the side surface29 and the interface between the first electrode 4 and the basesubstrate 1. Accordingly, insulation breakdown caused by insufficientinsulation of the side portion of the first electrode 4 and the vicinitythereof by the insulation layer 16 does not influence the performance ofthe two-terminal device 14, and therefore a desirable I-V characteristicis obtained.

Since the ZnS layer 3 is formed only in the hole 19, namely, in the areaof the two-terminal device 14, the inconveniences such as increase inthe leak current and fluctuation in impedance in accordance with thechange in the applied voltage caused by the ZnS layer 3 on the areaother than the two-terminal device 14 can be avoided.

Further, since the insulation layer 16 covers the entire top surface ofthe base substrate 1, the flatness of the base substrate 1 is improved.Such improved flatness allows the liquid crystal molecules in the LCDapparatus 36 to be easily aligned. In the case that the two-terminaldevice 14 is used in a reflection type LCD apparatus, the lightscattering characteristic of the LCD apparatus 36 is enhanced.

EXAMPLE 11

In an eleventh example according to the present invention, anothermethod for producing the LCD apparatus 36 illustrated in FIGS. 18 and 19will be described with reference to FIG. 21.

FIG. 21 is a schematic illustration of a part of a process for producingthe LCD apparatus 36 for solving the problem of insufficient insulationof the first electrode 4 by the ZnS layer 3.

First, a film of Ta is formed on the base substrate 1 in a specifiedthickness by sputtering and patterned into a specified pattern to formthe scanning line 2 having the first electrode 4 (part (a)). Thethickness of the scanning line 2 is 300 nm in the eleventh example. Thescanning line 2 having the first electrode 4 may be formed of aconductive thin film of titanium, molybdenum, aluminum, copper, ITO orthe like instead of Ta. As an etchant for patterning, a mixed solutionof hydrofluoric acid and nitric acid is used.

The ZnS layer 3 is formed on the base substrate 1 to cover the firstelectrode 2 by sputtering in a thickness of 200 nm (part (b)). Then, theZnS layer 3 is patterned into a specified pattern using the photoresist17 and the mask 18 (parts (c) and (d)). Namely, a part of thephotoresist 17 and a part of the ZnS layer 3 exposed to light areremoved. As the photoresist 17, a positive photoresist OFPR-800 producedby Tokyo Ohka Kabushiki Kaisha is used in the eleventh example, but anegative photoresist may be used. As an etchant for patterning the ZnSlayer 3 after development, hydrochloric acid is used.

Then, a film of SiO_(x) for forming the insulation layer 16 is formed onthe base substrate 1 to cover the photoresist 17 by sputtering in athickness of 1.0 μm, the ZnS layer 3 and the first electrode 4 (part(e)). The insulation layer 16 may be formed of an insulating materialsuch as SiN_(x) or a photoresist.

The photoresist 17 and the SiO_(x) layer on the photoresist 17 arepeeled off by a lift-off method (part (f)) to form the insulation layer16 having the hole 19 at a portion corresponding to the ZnS layer 3. Theinsulation layer 16 covers the base substrate 1 and the first electrode4 except for an area on which the ZnS layer 3 is located. In otherwords, the ZnS layer 3 is only in the hole 19 of the insulation layer16, namely, in the area of the two-terminal device 14. As thepeeling-off liquid, OMR-710 produced by Tokyo Ohka Kabushiki Kaisha isused.

A film of ITO is formed on the insulation layer 16 to cover the ZnSlayer 3 by sputtering to a thickness of 200 nm, and patterned to formthe pixel electrode 5 having the second electrode 8 (part (g)). Thepixel electrode 5 having the second electrode 8 may be formed of aconductive material such as aluminum, tantalum, titanium, molybdenum orcopper instead of ITO. As an etchant for patterning the ITO film,hydrobromic acid is used.

The two-terminal device 14 produced in the above-described mannerincludes a flat portion of the first electrode 4 but excludes a sideportion and the vicinity thereof including an edge formed at thejunction of a top surface 28 and a side surface 29 of the firstelectrode 4 and another edge formed at the junction of the side surface29 and the interface between the first electrode 4 and the basesubstrate 1. Accordingly, insulation breakdown caused by insufficientinsulation of the side portion of the first electrode 4 and the vicinitythereof by the insulation layer 16 does not influence the performance ofthe two-terminal device 14, and therefore a desirable I-V characteristicis obtained.

Since the ZnS layer 3 is formed only in the hole 19, namely, in the areaof the two-terminal device 14, the inconveniences such as increases inthe leak current and fluctuations in impedance in accordance with thechange in the applied voltage caused by the ZnS layer 3 formed on thearea other than the two-terminal device 14 can be avoided.

Further, since the insulation layer 6 covers the entire top surface ofthe base substrate 1, the flatness of the base substrate 1 is improved.Such improved flatness allows the liquid crystal molecules in the LCDapparatus 36 to be easily aligned. In the case that the two-terminaldevice 14 is used in a reflection type LCD apparatus 36 is enhanced.

EXAMPLE 12

In a twelfth example according to the present invention, a two-terminaldevice for a display apparatus and a method for producing the same forsolving the problem of insufficient insulation of the first electrode 4by the ZnS layer 3 will be described with reference to FIGS. 22 and 23.

FIG. 22 is a partial cross sectional view of an LCD apparatus 37 in thetwelfth example. A top view of the LCD apparatus 37 is similar to thatof the LCD apparatus 36 shown in FIG. 18.

As is shown in FIG. 22, the LCD apparatus 37 includes the basesubstrate 1. The scanning line 2 (not shown) is on the top surface ofthe base substrate 1, and the first electrode 4 is branched from thescanning line 2. The ZnS layer 3 acting as an active layer is on aspecified area of the first electrode 4. It should be noted that the ZnSlayer 3 is not on the entire base substrate 1. The insulation layer 16is on the base substrate 1, covering the first electrode 4 except for anarea on which the ZnS layer 3 is located. The insulation layer 16 has agreater thickness than that of the ZnS layer 3. A second electrode 25formed of a conductive thin film is on the ZnS layer 3. The pixelelectrode 5 is on the insulation layer 16, covering the second electrode25. The two-terminal device 14 includes the three-layer structureincluding the first electrode 4, the ZnS layer 3 and the secondelectrode 25.

The LCD apparatus 37 having the above-described structure is produced inthe following manner.

FIG. 23 is a schematic illustration of a part of the process forproducing the LCD apparatus 37.

First, a film of Ta is formed on the base substrate 1 in a specifiedthickness by sputtering and patterned into a specified pattern so as toform the scanning line 2 having the first electrode 4 (part (a)). Thethickness of the scanning line 2 is 300 nm in the twelfth example. Thescanning line 2 having the first electrode 4 may be formed of aconductive thin film of titanium, molybdenum, aluminum, copper, ITO orthe like instead of Ta. As an etchant for patterning, a mixed solutionof hydrofluoric acid and nitric acid is used.

Then, the insulation layer 16 is formed as is illustrated in parts (b)through (d).

A film of SiO_(x) is formed on the base substrate 1 to cover the firstelectrode 4 in a thickness of 1.0 μm (part (b)). Then, the film ofSiO_(x) is patterned into a specified pattern using a photoresist 17 anda mask 18 (parts (c) and (d)). In this way, the insulation layer 16having the hole 19 at the portion corresponding to the two-terminaldevice 14 is formed. As the photoresist 17, a positive photoresistOFPR-800 produced by Tokyo Ohka Kabushiki Kaisha is used in the tenthexample, but a negative photoresist may be used. The patterning afterdevelopment is performed by dry etching. The insulation layer 16 may beformed of an insulating material such as SiN_(x), Al₂ O₃, or AlN insteadof SiO_(x).

Then, the ZnS layer 3 is formed on the photoresist 17 to cover the firstelectrode 4 by sputtering in a thickness of 200 nm (part (e)).

A conductive thin film for forming the second electrode 25 is formed onthe ZnS layer 3 by sputtering to a thickness of 800 nm (part (f)). Inthe twelfth example, titanium is used for the conductive thin film.Instead of titanium, aluminum, ITO, tantalum, molybdenum, copper or thelike may be used.

The photoresist 17, and a part of the ZnS layer 3 and a part of theconductive film both corresponding to the photoresist 17 are peeled offby a lift-off method to keep the ZnS layer 3 only in the hole 19 andalso to form the second electrode 25 only in the hole 19 (part (g)). Inother words, the ZnS layer 3 and the second electrode 3 are formed onlyin the area of the two-terminal device 14. As the peeling-off liquid,OMR-710 produced by Tokyo Ohka Kabushiki Kaisha is used.

A film of ITO is formed on the insulation layer 16 to cover the secondelectrode 25 by sputtering to a thickness of 200 nm and patterned toform the pixel electrode 5 (part (h)). The pixel electrode 5 may beformed of a conductive material such as aluminum, tantalum, titanium,molybdenum or copper instead of ITO. As an etchant for patterning theITO film, a mixed solution of hydrofluoric acid and nitric acid is used.

In the twelfth example, the process for producing the LCD apparatus 37requires the step of forming the second electrode 25 in addition to theproduction process of the LCD apparatus 36 described in the tenth andthe eleventh examples. However, since the second electrode 25 and thepixel electrode 5 are formed of different materials from each other, thematerials for the second electrode 25 and the pixel electrode 5 can bechosen from a wider selection of materials than in the tenth and theeleventh examples.

EXAMPLE 13

In a thirteenth example according to the present invention, anothermethod for producing the LCD apparatus 37 illustrated in FIG. 22 will bedescribed with reference to FIG. 24.

FIG. 24 is a schematic illustration of a part of a process for producingthe LCD apparatus 37 for solving the problem of insufficient insulationof the first electrode 4 by the ZnS layer 3.

First, a film of Ta is formed on the base substrate 1 in a specifiedthickness by sputtering and patterned into a specified pattern to formthe scanning line 2 having the first electrode 4 (part (a)). Thethickness of the scanning line 2 is 300 nm in the thirteenth example.The scanning line 2 having the first electrode 4 may be formed of aconductive thin film of titanium, molybdenum, aluminum, copper, ITO orthe like instead of Ta. As an etchant for patterning, a mixed solutionof hydrofluoric acid and nitric acid is used.

The ZnS layer 3 is formed on the base substrate 1 to cover the firstelectrode 2 by sputtering in a thickness of 200 nm (part (b)). Then, aconductive thin film for forming the second electrode 25 is formed onthe ZnS layer 3 by sputtering to a thickness of 800 nm (part (c)). Theconductive thin film is formed of titanium. Instead of titanium,aluminum, ITO, tantalum, molybdenum, copper or the like may be used forthe conductive thin film.

The ZnS layer 3 and the titanium film are patterned into a specifiedpattern using the photoresist 17 and the mask 18 (parts (d) and (e)).Namely, a part of the photoresist 17, a part of the ZnS layer 3 and apart of the titanium film exposed to light are removed. In this mannerthe titanium film is patterned to be the second electrode 25. As thephotoresist 17, a positive photoresist OFPR-800 produced by Tokyo OhkaKabushiki Kaisha is used in the eleventh example, but a negativephotoresist may be used. As an etchant for patterning the titanium filmafter development, a mixture liquid of an aqueous solution of hydrogenperoxide and an aqueous solution of ammonia is used. As an etchant forpatterning the ZnS layer 3 after development, hydrochloric acid is used.

Then, a film of SiO_(x) for forming the insulation layer 16 is formed onthe base substrate 1 to cover the photoresist 17, the second electrode25, the ZnS layer 3 and the first electrode 4 by sputtering in athickness of 1.0 μm (part (f)). The insulation layer 16 may be formed ofan insulating material such as SiN_(x) or a photoresist.

The photoresist 17 and the SiO_(x) film on the photoresist 17 are peeledoff by a lift-off method (part (g)) to form the insulation layer 16enclosing the ZnS layer 3 and the second electrode 25. The insulationlayer 16 covers the base substrate 1 and the first electrode 4 exceptfor an area on which the ZnS layer 3 and the second electrode 25 arelocated. In other words, the ZnS layer 3 is only in the area of thetwo-terminal device 14. As for the peeling-off liquid, OMR-710 producedby Tokyo Ohka Kabushiki Kaisha is used.

A film of ITO is formed on the insulation layer 16 to cover the secondelectrode 25 by sputtering to a thickness of 200 nm, and patterned toform the pixel electrode 5 (part (h)). The pixel electrode 5 may beformed of a conductive material such as aluminum, tantalum, titanium,molybdenum or copper instead of ITO. As an etchant for patterning theITO film, hydrobromic acid is used.

In the thirteenth example, the process for producing the LCD apparatus37 requires the step of forming the second electrode 25 in addition tothe production process of the LCD apparatus 36 described in the tenthand the eleventh examples. However, since the second electrode 25 andthe pixel electrode 5 are formed of different materials from each other,the materials for the second electrode 25 and the pixel electrode 5 canbe chosen from a wider selection of materials than in the tenth and theeleventh examples.

EXAMPLE 14

Referring to FIG. 25, a fourteenth example of a display apparatusaccording to the present invention will be described.

FIG. 25 is a partial cross sectional view of an LCD apparatus 38 in thefourteenth example according to the present invention.

The LCD apparatus 38 is produced in the following manner.

The production process of the LCD apparatus 38 is identical as theproduction process described in the first example until the ZnS layer 3is formed. After the formation of the ZnS layer 3, an insulation layer26 is formed of a photosensitive resin. Instead of the photosensitiveresin, an inorganic oxide such as SiO₂ or Al₂ O₅, an inorganic nitridesuch as SiN_(x) or AlN, or an organic substance such as acrylic resins,polyimide, or polyurea may be used for the insulation layer 26. A hole27 is formed in the insulation layer 26, at a portion corresponding tothe two-terminal device 14. A part of a surface of the insulation layer26 corresponding to the pixel is corrugated by a photographic process.

A conductive film is formed on the insulation layer 26 and patternedinto a specified pattern to form the pixel electrode 5 having the secondelectrode 8. The pixel electrode 5 may be formed of Al, Ag, Cr, Ni, Cu,Ti or alloys thereof.

A top part of the hole 27 has an area of 10 μm² to 1,000 μm² inclusive.In the case that the area of the top part of the hole 27 is also toolarge, the capacitance of the two-terminal device 14 is too large, andthus the ratio of the capacitance of the liquid crystal layer 10 withrespect to the capacitance of the two-terminal device 14 (C_(L) /C_(D))is too small. Accordingly, the two-terminal device 14 is not suppliedwith a voltage at a sufficient level and is not conductive. As a result,the liquid crystal layer 10 is not supplied with a voltage at asufficient level. By contrast, as the area of the top part of the hole27 is too small, the ratio of the capacitance of the liquid crystallayer 10 with respect to the capacitance of the two-terminal device 14(C_(L) /C_(D)) is sufficiently large. However, the amount of the currentflowing through the liquid crystal layer 10 is too small, and thus theliquid crystal layer 10 is not sufficiently charged. For these reasons,it is important to choose an appropriate area of the top part of thehole 27.

After the formation of the pixel electrode 5, a material for thealignment film 9 is formed to cover the insulation layer 26 and thepixel electrode 5. The material is then cured and aligned to form thealignment film 9. In this manner, the lamination on the base substrate 1is completed.

On an insulating counter substrate 12, counter electrodes 13 are formedin stripes in such a direction as to cross the pixel electrode 5. Amaterial for the alignment film 11 is formed on the counter substrate 12to cover the counter electrodes 13. The material is then cured andaligned to form the alignment film 11. In this manner, the lamination onthe counter substrate 12 is completed.

The lamination on the base substrate 1 and the lamination on the countersubstrate 12 are assembled together with spacers (not shown) sandwichedtherebetween, and the liquid crystal is injected between the twolaminations to form the liquid crystal layer 10. Thus, the LCD apparatus38 is produced. In the fourteenth example, White-Taylor guest-hostliquid crystal is used.

In the fourteenth example, the pixel electrode 5 is formed of metal inorder to be suitable for a reflection type LCD apparatus. Since thesurface of the insulation layer 26 is corrugated, a surface of the pixelelectrode 5 formed on the insulation layer 26 is also corrugated. Bysuch corrugation, the light incident on the LCD apparatus 38 is notreflected in such a manner as is reflected on a mirror surface, and thusthe display quality is enhanced.

The White-Taylor guest host liquid crystal does not require a deflectionplate, and therefore displays brighter images than the conventionaltwisted nematic liquid crystal when used in a reflection-type LCDapparatus. Even in a transmission type LCD apparatus, the White-Taylorguest host liquid crystal displays brighter images than the conventionaltwisted nematic liquid crystal.

The White-Taylor liquid crystal requires a high driving voltage.Accordingly, satisfactory displays cannot be obtained in an LCDapparatus including the conventional two-terminal device using Ta₂ O₅ orSiN_(X) in the case that the White-Taylor liquid crystal is used as thedisplay medium. By using a two-terminal device using ZnS according tothe present invention, satisfactory display is obtained even if theWhite-Taylor liquid crystal is used as the display medium.

EXAMPLE 15

In a fifteenth example according to the present invention, theinsulation layer 26 of the LCD apparatus 38 is formed of an insulatingcolor photoresist. Except for the material of the insulation layer 26,the LCD apparatus 38 is identical in structure with the LCD apparatus inthe fourteenth example. As the color photoresist, for example, the ColorMosaic CK produced by Fuji Hunt Electronics Technology Kabushiki Kaishais used.

The LCD apparatus 38 in the fifteenth example is produced in thefollowing manner.

After the ZnS layer 3 is formed in the manner described in the firstexample, a layer of the color photoresist is formed on the ZnS layer 3to a thickness of approximately 300 nm by a spinner. Then, the hole 27is formed in the insulation layer 26 and the surface of the insulationlayer 26 is corrugated in the manner described in the fourteenthexample. After that, the production process of the LCD apparatus 38 inthe fifteenth example is also identical with the production processdescribed in the fourteenth example.

Due to the color photoresist used for the insulation layer 26, the lightincident on an area other than the pixel is absorbed by the insulationlayer 26. In other words, the insulation layer 26 acts as a blackmatrix, and thus the display contrast is improved.

As has been described so far, according to the present invention, thetwo-terminal device of the LCD apparatus includes an insulation layer(active layer) formed of ZnS, which has an I-V characteristic showing acontinuous curve. Accordingly, a two-terminal having an I-Vcharacteristic which is satisfactory both in steepness and symmetry andis excellent in nonlinearity can be obtained. A display apparatusincluding such a two-terminal device realizes high quality display.

In the case that the ZnS layer is heated, the uniformity in quality andthe stability of the two-terminal device is improved. A displayapparatus including such a two-terminal device has few defects in imagesand little dispersion in display quality among the pixels.

The ZnS layer is formed only on a flat portion of the surface of thefirst electrode. Accordingly, the side portion of the first electrodeand the vicinity thereof including the edges of the first electrode isexcluded from the two-terminal device. Due to such a structure, theproblem caused in, for example, a conventional MIM element, namely,insulation breakdown caused by insufficient insulation of the sideportion of the first metal layer and the vicinity thereof is solved.

In the case that the ZnS layer is formed only in the area of thetwo-terminal device, increase in the leak current and fluctuation inimpedance in accordance with the change in the applied voltage which arecaused by the ZnS layer formed in an area other than the two-terminaldevice is avoided.

By implanting various impurities into the ZnS layer during the formationof the ZnS layer, the I-V characteristic of the two-terminal device caneasily be changed. Utilizing this fact, a two-terminal device having anoptimum I-V characteristic in accordance with the display medium, thepurpose for display, or the type of the display apparatus to be producedcan be obtained.

Various other modifications will be apparent to and can be readily madeby those skilled in the art without departing from the scope and spiritof this invention. Accordingly, it is not intended that the scope of theclaims appended hereto be limited to the description as set forthherein, but rather that the claims be broadly construed.

What is claimed is:
 1. A display apparatus, comprising:a firstsubstrate; a pixel electrode formed on the first substrate; a scanningline for carrying a signal to the pixel electrode for driving the pixelelectrode; a switching device for receiving the signal from the scanningline and switching the pixel electrodes into one of a conductive stateand a non-conductive state in accordance with the signal; a secondsubstrate opposed to the first substrate; a counter electrode on thesecond substrate; and a display medium layer sandwiched between thefirst substrate and the second substrate; wherein the switching deviceincludes a two-terminal element having:a first electrode which is a partof the scanning line; a zinc sulfide layer on the first electrode, saidzinc sulfide layer having an I-V characteristic expressed by acontinuous curve, said I-V characteristic having controlled steepnessand symmetry; and a second electrode located on the zinc sulfide layerand electrically connected to the pixel electrode.
 2. A displayapparatus according to claim 1, wherein the zinc sulfide layer has athickness of 10 nm to 1 μm.
 3. A display apparatus according to claim 1,wherein the zinc sulfide layer is on an entire surface of the firstsubstrate, covering the scanning line.
 4. A display apparatus accordingto claim 1, wherein the zinc sulfide layer is formed on a specified areaof the first electrode excluding an edge portion of the first electrode.5. A display apparatus according to claim 1, wherein the first electrodeis branched from the scanning line.
 6. A display apparatus according toclaim 5, wherein the second electrode is a specified area within thepixel electrode.
 7. A display apparatus according to claim 1, whereinthe first electrode is a specified area within the scanning line.
 8. Adisplay apparatus according to claim 1, wherein the second electrode andthe pixel electrode are formed of an identical material.
 9. A displayapparatus according to claim 1, wherein the second electrode is branchedfrom the pixel electrode.
 10. A display apparatus according to claim 1,further comprising a first insulation layer sandwiched between the firstelectrode and the zinc sulfide layer.
 11. A display apparatus accordingto claim 10, further comprising a second insulation layer sandwichedbetween the zinc sulfide layer and the second electrode.
 12. A displayapparatus according to claim 11, wherein the second insulation layer hasa hole, through which the second electrode and the zinc sulfide layerare electrically connected with each other.
 13. A display apparatusaccording to claim 12, wherein the hole has an opening having an area of10 μm² to 1,000 μm² inclusive.
 14. A display apparatus according toclaim 12, wherein the second insulation layer has a corrugated surface.15. A display apparatus according to claim 14, wherein the secondinsulation layer is formed of a color photoresist.
 16. A displayapparatus according to claim 15, wherein the display medium is aWhite-Taylor guest-host liquid crystal.
 17. A display apparatusaccording to claim 11, wherein the second insulation layer is formed ofa substance selected from the group consisting of nitrogen compounds andsilicon oxide.
 18. A display apparatus according to claim 7, wherein atleast one of the pixel electrodes, the first electrode and the secondelectrode is formed of a transparent conductive layer.
 19. A displayapparatus according to claim 11, wherein the second insulation layerincludes a plurality of insulation layers formed of differentsubstances.
 20. A display apparatus according to claim 1, furthercomprising a first insulation layer sandwiched between the firstelectrode and the zinc sulfide layer and a second insulation layerbetween the zinc sulfide layer and the second electrode.
 21. A displayapparatus according to claim 20, wherein the first insulation layer andthe second insulation layer are formed of an identical material witheach other.
 22. A display apparatus according to claim 20, wherein thesecond insulation layer has a hole, through which the second electrodeand the zinc sulfide layer are electrically connected with each other.23. A display apparatus according to claim 22, wherein the hole has anopening having an area of 10 μm² to 1,000 μm² inclusive.
 24. A displayapparatus according to claim 22, wherein the second insulation layer hasa corrugated surface.
 25. A display apparatus according to claim 24,wherein the second insulation layer is formed of a color photoresist.26. A display apparatus according to claim 25, wherein the displaymedium is a White-Taylor guest-host liquid crystal.
 27. A displayapparatus according to claim 20, wherein the first insulation layer andthe second insulation layer are each formed of a substance selected fromthe group consisting of nitrogen compounds and silicon oxide.
 28. Adisplay apparatus according to claim 27, wherein at least one of thepixel electrodes, the first electrode and the second electrode is formedof a transparent conductive layer.
 29. A display apparatus according toclaim 20, wherein the first insulation layer and the second insulationlayer each includes a plurality of insulation layers formed of differentsubstances.
 30. A display apparatus according to claim 1, wherein thezinc sulfide layer includes an impurity.
 31. A display apparatusaccording to claim 30, wherein the impurity is selected from the groupconsisting of manganese, copper, rare earth elements, compoundsincluding a rare earth element, and the III-group elements.
 32. Adisplay apparatus according to claim 1, wherein the zinc sulfide layeris formed so as to have a composition ratio expressed by 1>x>0.5 wherethe zinc sulfide layer has a composition expressed by Zn_(x)S.sub.(1-x).
 33. A display apparatus according to claim 1, wherein thepixel electrode is formed of a substance selected from the groupconsisting of Al, Ag, Cr, Ni, Cu, Ti and alloys thereof.
 34. A displayapparatus according to claim 1, wherein the display medium is a liquidcrystal.
 35. A display apparatus according to claim 1, furthercomprising a plurality of insulation layers on the first substrate forsupplying the first substrate with an insulating property.
 36. A displayapparatus comprising:a first substrate; a pixel electrode formed on thefirst substrate; a scanning line for carrying a signal to the pixelelectrode for driving the pixel electrode; a switching device forreceiving the signal from the scanning line and switching the pixelelectrodes into one of a conductive state and a non-conductive state inaccordance with the signal; a second substrate opposed to the firstsubstrate; a counter electrode on the second substrate; and a displaymedium layer sandwiched between the first substrate and the secondsubstrate; wherein the switching device includes a two-terminal elementhaving:a first electrode which is a part of the scanning line; a zincsulfide layer formed on a specified area of the first electrodeexcluding an edge portion of the first electrode; a second electrodelocated on the zinc sulfide layer and electrically connected to thepixel electrode; and an insulation layer sandwiched between the pixelelectrode and the first substrate except for the specified area on whichthe zinc sulfide layer is located.
 37. A display apparatus comprising:afirst substrate; a pixel electrode formed on the first substrate; ascanning line for carrying a signal to the pixel electrode for drivingthe pixel electrode; a switching device for receiving the signal fromthe scanning line and switching the pixel electrodes into one of aconductive state and a non-conductive state in accordance with thesignal; a second substrate opposed to the first substrate; a counterelectrode on the second substrate; and a display medium layer sandwichedbetween the first substrate and the second substrate; wherein theswitching device includes a two-terminal element having:a firstelectrode which is a part of the scanning line; a zinc sulfide layer onthe first electrode, said zinc sulfide layer having an I-Vcharacteristic expressed by a continuous curve; a second electrodelocated on the zinc sulfide layer and electrically connected to thepixel electrode; and a first insulation layer sandwiched between thefirst electrode and the zinc sulfide layer, wherein the first insulationlayer is obtained by anodizing the first electrode.
 38. A displayapparatus comprising:a first substrate; a pixel electrode formed on thefirst substrate; a scanning line for carrying a signal to the pixelelectrode for driving the pixel electrode; a switching device forreceiving the signal from the scanning line and switching the pixelelectrodes into one of a conductive state and a non-conductive state inaccordance with the signal; a second substrate opposed to the firstsubstrate; a counter electrode on the second substrate; and a displaymedium layer sandwiched between the first substrate and the secondsubstrate; wherein the switching device includes a two-terminal elementhaving:a first electrode which is a part of the scanning line; a zincsulfide layer on the first electrode, said zinc sulfide layer having anI-V characteristic expressed by a continuous curve; a second electrodelocated on the zinc sulfide layer and electrically connected to thepixel electrode and a first insulation layer sandwiched between thefirst electrode and the zinc sulfide layer and a second insulation layerbetween the zinc sulfide layer and the second electrode, wherein thefirst insulation layer is obtained by anodizing the first insulationlayer.
 39. A display apparatus comprising:a first substrate; a pixelelectrode formed on the first substrate; a scanning line for carrying asignal to the pixel electrode for driving the pixel electrode; aswitching device for receiving the signal from the scanning line andswitching the pixel electrodes into one of a conductive state and anon-conductive state in accordance with the signal; a second substrateopposed to the first substrate; a counter electrode on the secondsubstrate; and a display medium layer sandwiched between the firstsubstrate and the second substrate; wherein the switching deviceincludes a two-terminal element having:a first electrode which is a partof the scanning line; a zinc sulfide layer on the first electrode, saidzinc sulfide layer having an I-V characteristic expressed by acontinuous curve; a second electrode located on the zinc sulfide layerand electrically connected to the pixel electrode; and a secondinsulation layer sandwiched between the zinc sulfide layer and thesecond electrode, wherein the first insulation layer is formed of asubstance selected from the group consisting of nitrogen compounds andsilicon oxide.
 40. A display apparatus according to claim 39, wherein atleast one of the pixel electrodes, the first electrode and the secondelectrode is formed of a transparent conductive layer.
 41. A displayapparatus comprising:a first substrate; a pixel electrode formed on thefirst substrate; a scanning line for carrying a signal to the pixelelectrode for driving the pixel electrode; a switching device forreceiving the signal from the scanning line and switching the pixelelectrodes into one of a conductive state and a non-conductive state inaccordance with the signal; a second substrate opposed to the firstsubstrate; a counter electrode on the second substrate; and a displaymedium layer sandwiched between the first substrate and the secondsubstrate; wherein the switching device includes a two-terminal elementhaving:a first electrode which is a part of the scanning line; a zincsulfide layer on the first electrode, said zinc sulfide layer having anI-V characteristic expressed by a continuous curve; a second electrodelocated on the zinc sulfide layer and electrically connected to thepixel electrode; and a first insulation layer sandwiched between thefirst electrode and the zinc sulfide layer, wherein the first insulationlayer includes a plurality of insulation layers formed of differentsubstances.